DSL building patterns in Ruby

Introduction

A DSL (Domain Specific Language) helps you write code that reads like plain English for a specific task. Building DSLs makes your code easier to understand and use for that task.

You want to create a simple way for users to configure settings without complex code.

You need a readable way to describe workflows or rules in your program.

You want to hide complicated logic behind easy-to-use commands.

You are building a testing framework that reads like natural language.

You want to make your code more expressive and self-explanatory.

Syntax

Ruby

class MyDSL
  def initialize(&block)
    instance_eval(&block) if block_given?
  end

  def command(arg)
    # do something with arg
  end
end

MyDSL.new do
  command 'example'
end

Use instance_eval to run the block in the context of the DSL object.

Define methods that represent commands or keywords in your DSL.

Examples

This DSL lets you say hello in a simple way.

Ruby

class GreetingDSL
  def initialize(&block)
    instance_eval(&block) if block_given?
  end

  def say(text)
    puts "Hello, #{text}!"
  end
end

GreetingDSL.new do
  say 'world'
end

This DSL collects settings in a hash for easy access.

Ruby

class ConfigDSL
  attr_reader :settings

  def initialize(&block)
    @settings = {}
    instance_eval(&block) if block_given?
  end

  def set(key, value)
    @settings[key] = value
  end
end

config = ConfigDSL.new do
  set :color, 'blue'
  set :size, 10
end

puts config.settings

This DSL defines tasks and runs them in order.

Ruby

class TaskDSL
  def initialize(&block)
    @tasks = []
    instance_eval(&block) if block_given?
  end

  def task(name, &block)
    @tasks << {name: name, action: block}
  end

  def run_all
    @tasks.each { |t| t[:action].call }
  end
end

workflow = TaskDSL.new do
  task 'clean' do
    puts 'Cleaning...'
  end
  task 'build' do
    puts 'Building...'
  end
end

workflow.run_all

Sample Program

This program builds a simple DSL to collect messages and then print them.

Ruby

class SimpleDSL
  def initialize(&block)
    @messages = []
    instance_eval(&block) if block_given?
  end

  def add_message(text)
    @messages << text
  end

  def show_messages
    @messages.each { |msg| puts "Message: #{msg}" }
  end
end

my_dsl = SimpleDSL.new do
  add_message 'Hello'
  add_message 'DSL'
  add_message 'World'
end

my_dsl.show_messages

Important Notes

Use instance_eval carefully because it changes the context to the DSL object.

Keep DSL commands simple and focused on the domain task.

Test your DSL with real examples to make sure it reads naturally.

Summary

DSLs make code easier to read and write for specific tasks.

Use instance_eval to run blocks inside your DSL object.

Define clear methods that represent your DSL commands.

Practice

1. What is the main purpose of using a DSL (Domain Specific Language) in Ruby?

easy

A. To make code easier to read and write for specific tasks

B. To speed up Ruby interpreter execution

C. To replace Ruby's core syntax completely

D. To automatically generate user interfaces

Solution

1. Step 1: Understand DSL purpose

   A DSL is designed to simplify and clarify code for a specific domain or task.

2. Step 2: Compare options

   Only To make code easier to read and write for specific tasks correctly states the purpose of DSLs; others describe unrelated goals.

3. Final Answer:

   To make code easier to read and write for specific tasks -> Option A

4. Quick Check:

   DSL purpose = simplify domain code [OK]

Hint: DSLs focus on specific tasks for clearer code [OK]

Common Mistakes:

• Thinking DSLs speed up Ruby interpreter
• Believing DSLs replace Ruby syntax entirely
• Confusing DSLs with UI generators

2. Which Ruby method is commonly used to execute a block inside the context of a DSL object?

easy

A. eval_block

B. block_exec

C. class_eval

D. instance_eval

Solution

1. Step 1: Recall Ruby block execution methods

   Ruby provides methods like instance_eval and class_eval to run blocks in specific contexts.

2. Step 2: Identify the method for DSL context

   instance_eval runs the block inside the instance's context, which is typical for DSLs.

3. Final Answer:

   instance_eval -> Option D

4. Quick Check:

   Block runs in object context = instance_eval [OK]

Hint: Use instance_eval to run blocks inside DSL objects [OK]

Common Mistakes:

• Confusing class_eval with instance_eval
• Using non-existent methods like eval_block
• Assuming block_exec is a Ruby method

3. What will be the output of this Ruby DSL code?

class Builder
  def initialize
    @steps = []
  end
  def step(name)
    @steps << name
  end
  def build(&block)
    instance_eval(&block)
    @steps
  end
end

b = Builder.new
result = b.build do
  step 'clean'
  step 'compile'
  step 'test'
end
puts result.inspect

medium

A. [:clean, :compile, :test]

B. ["clean", "compile", "test"]

C. ["step", "step", "step"]

D. SyntaxError

Solution

1. Step 1: Understand the Builder class behavior

   The build method runs the block inside the Builder instance, collecting step names as strings in @steps.

2. Step 2: Trace the block execution

   Each call to step adds the string name to @steps: 'clean', 'compile', 'test'. The build method returns @steps.

3. Final Answer:

   ["clean", "compile", "test"] -> Option B

4. Quick Check:

   Collected steps = ["clean", "compile", "test"] [OK]

Hint: instance_eval runs block adding strings to array [OK]

Common Mistakes:

• Expecting symbols instead of strings
• Thinking step method returns step name
• Confusing array contents with method names

4. Identify the error in this DSL builder code:

class Configurator
  def set(name, value)
    @settings ||= {}
    @settings[name] = value
  end
  def configure(&block)
    instance_eval(block)
  end
end

c = Configurator.new
c.configure do
  set :mode, 'fast'
end

medium

A. Wrong argument passing to instance_eval

B. Missing @settings initialization

C. set method missing return value

D. Block not passed to configure method

Solution

1. Step 1: Check instance_eval usage

   instance_eval expects a block passed with &block, but here it is called as instance_eval(block) without &.

2. Step 2: Understand correct block passing

   The correct call is instance_eval(&block) to convert the block argument to a block for instance_eval.

3. Final Answer:

   Wrong argument passing to instance_eval -> Option A

4. Quick Check:

   Use &block with instance_eval [OK]

Hint: Pass block with & when calling instance_eval [OK]

Common Mistakes:

• Calling instance_eval without & for block
• Forgetting to initialize @settings
• Expecting set to return a value

5. You want to build a Ruby DSL that allows nested configuration like this:

config = Config.new do
  database do
    adapter 'sqlite3'
    pool 5
  end
  cache do
    enabled true
    size 64
  end
end

Which approach best supports this nested DSL pattern?

hard

A. Use global variables to store settings inside blocks

B. Use class variables to share state across blocks

C. Use instance_eval with separate classes for database and cache blocks

D. Use eval with string interpolation for nested blocks

Solution

1. Step 1: Understand nested DSL needs

   Nested blocks require separate context objects for database and cache to hold their settings.

2. Step 2: Identify best practice

   Using instance_eval on separate classes for each nested block cleanly isolates configuration and supports nested DSL.

3. Final Answer:

   Use instance_eval with separate classes for database and cache blocks -> Option C

4. Quick Check:

   Nested DSL = instance_eval + separate classes [OK]

Hint: Use instance_eval with helper classes for nested blocks [OK]

Common Mistakes:

• Using global variables causing conflicts
• Using class variables mixing state
• Using eval with strings risking security and complexity